1. Field of the Invention
This invention relates generally to crystal oscillators, and especially to oscillators able to work under adverse environmental conditions, such as high temperature, shocks and/or vibrations.
2. Description of the Related Art
Crystal oscillators are commonly used as time measurement or time base devices. Crystal oscillators usually include a plane element made e.g. of a piezoelectric crystal, and able, when subjected to an exciting voltage, to oscillate at a given frequency. The resulting oscillating signal may be used for any signal processing or measurement related to time. The reliability of such measurement depends on the stability of the oscillator, which is greatly influenced by environmental conditions, such as temperature and mechanical stress (vibrations and shocks). The crystal element is usually fixed parallel to and on a base so that it may freely oscillate, while being electrically connected to the other electronic components of the oscillator. Shocks and/or vibrations may damage or break the connection between the crystal and the base, or break the crystal itself. Extreme temperature or rapid temperature changes, besides any drift of the oscillating frequency, may have the same consequences due to differential thermal expansion between the oscillating element and the base.
The connection between the oscillating crystal element and the base of the oscillator is thus critical.
Among the numerous technical areas where crystal oscillators are used, are well logging techniques and measurement while drilling techniques (hereafter referred to as MWD), wherein a tool is lowered in a well to carry out physical measurements from which are derived information related to earth formations surrounding the well and downhole drilling conditions. The problem of stability hereabove referred to is all the more of concern in the logging and MWD techniques since the environmental conditions are particularly severe in a well.
Known crystal oscillators are usually built in such a way that the crystal is rigidly mounted on the base, by glue for instance. The rigid-type connection enhances the risks of excessive performance changes and possible breakage of the crystal due to its brittleness, since shocks, vibrations and thermal differential expansion stress are thus integrally transmitted to the crystal.
It has been attempted, in a known manner, to obviate these difficulties by mounting the crystal on a base through flexible legs. However, this implementation has not been proving satisfactory since it increases the overall dimensions and manufacturing costs of the oscillator. The flexible legs allow the crystal to move too freely which leads to breakage of the crystal in the high shock and vibration environment of well logging and MWD.
Accordingly, there is a need for a crystal oscillator able to withstand shocks and vibrations as well as temperature changes.